The field of the present disclosure relates to production of polycrystalline silicon in a substantially closed-loop process and, particularly, processes that include decomposition of trichlorosilane produced from metallurgical grade silicon.
Polycrystalline silicon is a vital raw material used to produce many commercial products including, for example, integrated circuits and photovoltaic (i.e., solar) cells. Polycrystalline silicon is often produced by a chemical vapor deposition mechanism in which silicon is deposited from a thermally decomposable silicon compound onto silicon particles in a fluidized bed reactor or onto silicon rods as in a Siemens-type reactor. The seed particles continuously grow in size until they exit the reactor as polycrystalline silicon product (i.e., “granular” polycrystalline silicon). Suitable decomposable silicon compounds include, for example, silane and halosilanes such as trichlorosilane.
Trichlorosilane may be produced by contacting hydrogen chloride with a source of silicon as shown in the reaction below,Si+3HCl→SiHCl3+H2  (1),or by contacting silicon tetrachloride and hydrogen with a source of silicon as shown in the reaction below,Si+3SiCl4+2H2→4SiHCl3  (2).Hydrogen chloride and silicon tetrachloride are relatively expensive components in trichlorosilane-based production of polycrystalline silicon.
A continuing need exists for processes for producing polycrystalline silicon by thermal decomposition of trichlorosilane that reduce the amount of hydrogen and chlorine used relative to conventional methods and for methods that are capable of producing polycrystalline silicon in a substantially closed-loop process relative to hydrogen chloride. A continuing need also exists for systems for producing polycrystalline silicon that make use of such processes.